An ink jet printer or another conventional printer is fitted with a sheet feeder for feeding one by one the sheets of paper stacked on a sheet support. The feeder includes a rotatable shaft extending horizontally over the support. Fixed to the shaft are semi-cylindrical feed rollers for feeding the sheets one after one. The support is urged toward the rollers by an urging means. A rear portion of the support is covered normally with a cover. The shaft includes disks formed on it. A collar is fitted rotatably on the periphery of each disk. The collars on the disks are smaller slightly in diameter than the rollers. When the cylindrical surfaces of the rollers are away from the support, the collars are in contact with the top sheet so as to keep the sheet spaced at a predetermined distance from the axes of the rollers.
Some sheet feeders of this type each include a sheet cassette fitted with a cover and a sheet support which interlock or work with each other. When the cover opens, it pushes the right and left ends of the support, thereby moving the support against the force of the urging means away from the horizontal rotatable shaft. Consequently, when the cylindrical surfaces of the rollers are away from the support, the support is spaced from the collars so that sheets of paper can be stacked on the support. When the cover closes, the urging means returns the support toward the rollers. This brings the collars into contact with the top one of the sheets stacked on the support. Consequently, the sheet is spaced at a predetermined distance from the axes of the rollers. As a result, when the rollers turn, the sheets can be fed one by one.
The sheet support tends to curve in such a manner that its right and left ends are lower than its middle.
In general, a sheet support is made of synthetic resin, and includes walls protruding down from its periphery and/or ribs extending longitudinally and laterally on its back in order to improve its rigidity. It is not avoidable for the support to curve in such a manner that the support is convex upward due to the distortion or deformation of the support, which may be caused by the contraction of the peripheral walls after the support is molded.
As stated above, some sheet feeders each include a sheet cassette fitted with a cover and a sheet support which interlock with each other. When the cover opens and closes, the support moves. When the cassette is removed from the feed rollers, and packed and/or kept, the cover keeps pressing the right and left ends of the support urged in the opposite direction at a position or positions inward from these ends. The urging force curves the support in such a manner that the support is convex upward. If the support keeps curved for a long time while it is packed, it may be permanently curved.
When the cover is closed and the cylindrical surfaces of the feed rollers are away from the sheet support, with the support curved in such a manner that its right and left ends are lower than its middle, the only collars positioned near the middle are in contact with the top one of the sheets stacked on the support. As a result, the top sheet is spaced farther than the predetermined distance from the axes of the rollers. Therefore, the rollers may not be able to feed the sheets securely.